A chemical sensor is a device which monitors the concentration of a given chemical species in a liquid or a gas. Chemical sensors are used, for example, to detect unsafe levels of poisonous or explosive gases in the work and home environments.
Chemical sensors formed using hybrid technology, such as for example sensors formed on ceramic substrates, are well known. It is also known to fabricate a semiconductor chemical sensor on a semiconductor substrate. This invention is concerned with semiconductor chemical sensors.
Typically chemical sensors comprise a sensitive layer, which is sensitive to particular chemical species which are to be detected by the sensor. The reaction of the sensitive layer with the chemical species to be detected results in a change in the physical properties of the sensitive layer, e.g. resistivity or surface potential. As the reaction of the sensitive layer is governed by thermodynamic relations, temperature plays an important role in optimising the output of the sensor device, e.g. sensitivity and selectivity. Some sensors comprise a heater for increasing the temperature of the sensitive layer to increase the sensitivity and selectivity of the sensor. Depending on the chemical species to be detected, chemical sensors may need to be heated to quite high temperatures, for example in the range of 250.degree.-650.degree. C.
Since the selectivity of a chemical sensor depends on the temperature to which the sensitive layer is heated, it is therefore desirable that the temperature of the sensitive layer or heater is accurately monitored over a temperature range such as 250.degree.-650.degree. C.
Poly resistors and metallic resistors have been used to monitor the operating temperature in semiconductor chemical sensors, see for example the article by Wan Young Chun, in Sensors and Actuators B, 20 (1994) 139-143. However, such techniques both require significant die space and can only monitor the temperature over a large area of the die. Such techniques therefore lack accuracy.
It is therefore desirable to provide an improved semiconductor chemical sensor device with a temperature detector which does not suffer from the above referenced problems.